1. Field of the Invention
The present invention relates to voltage-controlled oscillators and communication apparatuses incorporating the same. More particularly, the present invention relates to voltage-controlled oscillators generating high frequency signals in different frequency bands and communication apparatuses incorporating the oscillators.
2. Description of the Related Art
Recently, multiple communication systems using different frequency bands have come into widespread use. The recent tendency has permitted a single communication apparatus to be used to handle multiple communication systems. In this case, in the communication apparatus, it is necessary for a high frequency oscillation circuit such as a voltage-controlled oscillator defining a local oscillation circuit to generate signals having different frequency bands according to the multiple communication systems.
FIG. 7 shows a circuit diagram of a conventional voltage-controlled oscillator. In this figure, there is shown a voltage-controlled oscillator 50 generating high frequency signals of two different frequency bands disclosed in Japanese Unexamined Patent Application Publication No. 10-163750. The voltage-controlled oscillator 50 includes first and second resonant circuits 51a and 51b, first and second oscillation circuits 52a and 52b oscillating at resonance frequencies of the first and second resonant circuits 51a and 51b, an amplifying circuit 53 amplifying oscillation signals of the first and second oscillation circuits 52a and 52b, a control terminal 54 applying a control voltage to each of the first and second resonant circuits 51a and 51b, power-source terminals 55a and 55b of the first and second oscillation circuits 52a and 52b, a power-source terminal 56 of the amplifying circuit 53, and an output terminal 57 outputting a high frequency signal.
The first resonant circuit 51a includes a variable-capacitance diode D51a, a stripline S51a defining a resonator, a frequency adjusting capacitor C51a, a coil L51a, and a capacitor C52a. The second resonant circuit 51b includes a variable-capacitance diode D51b, a stripline S51b defining a resonator, a frequency adjusting capacitor C51b, a coil L51b, and a capacitor C52b. The stripline S51a and the variable-capacitance diode D51a constitute a parallel resonant circuit, and the stripline S51b and the variable-capacitance diode D51b also constitute a parallel resonant circuit.
The first oscillation circuit 52a includes a transistor Q51a, capacitors C53a to C55a, and resistors R51a to R53a. The second oscillation circuit 52b includes a transistor Q51b, capacitors C53b to C55b, and resistors R51b to R53b. The amplifying circuit 53 includes a transistor Q52, a coil L52, capacitors C56 to C60, and resistors R54 to R56.
The first resonant circuit 51a is connected to the first oscillation circuit 52a via a coupling capacitor C61a, and the second resonant circuit 51b is connected to the second oscillation circuit 52b via a coupling capacitor C61b. The first and second oscillation circuits 52a and 52b are connected to the amplifying circuit 53 via coupling capacitors C62a and C62b. 
Next, a description will be provided of the operation of the voltage-controlled oscillator 50 shown in FIG. 7. When a power source is supplied to the power-source terminal 55a of the first oscillation circuit 52a and a power source is not supplied to the power-source terminal 55b of the second oscillation circuit 52b, only the first oscillation circuit 52a is driven whereas the second oscillation circuit 52b is not driven. In addition, when a power source is supplied to the amplifying circuit 53 from the power-source terminal 56, the amplifying circuit 53 is driven. In this case, in a range permitting oscillation of the first oscillation circuit 52a, a high frequency signal according to a direct current voltage applied by the control terminal 54 is output from the output terminal 57. In contrast, when a power source is supplied to the power-source terminal 55b of the second oscillation circuit 52b and a power source is not supplied to the power-source terminal 55a of the first oscillation circuit 52a, in a range permitting oscillation of the second oscillation circuit 52b, a high frequency signal according to a direct current voltage applied by the control terminal 54 is output from the output terminal 57.
The above-described operation can be performed by roughly coupling the first and second oscillation circuits 52a and 52b and the amplifying circuit 53 defining the voltage-controlled oscillator 50 via the coupling capacitors C62a and C62b having low capacitances so that no frequency change occurs even though the impedance of the output terminal 57 of the voltage-controlled oscillator 50 changes. For example, when the first oscillation circuit 52a is driven and the second oscillation circuit 52b is not driven, of the oscillation signals of the first oscillation circuit 52a, most of signals passing through the coupling capacitor C62a flow into the amplifying circuit 53 and hardly flow into the second oscillation circuit 52b, which is not driving, via the coupling capacitor C62b. In other words, since the coupling capacitors C62a and C62b are capacitors provided for rough coupling, as compared with the impedance of an input terminal of the amplifying circuit 53, each of the capacitors C62a and C62b has significantly high impedance. As a result, most of the oscillation signals of the first oscillation circuit 52a flow into the amplifying circuit 53 having low impedance. This is also applied to the case in which the second oscillation circuit 52b oscillates and the first oscillation circuit 52a does not oscillate. With this arrangement, without causing any mutual influence between the first and second oscillation circuits 52a and 52b, the first and second oscillation circuits 52a and 52b can be connected to the amplifying circuit 53.
However, the above conventional voltage-controlled oscillator requires a plurality of oscillation circuits to respond to different frequency bands. Thus, there is a problem in that the size of the voltage-controlled oscillator tends to be larger.
Furthermore, when characteristic deterioration occurs in the coupling capacitors connecting the plurality of oscillation circuits and the amplifying circuit, the oscillation signals of the driving oscillation circuit flow into the oscillation circuit that is not driving. As a result, the characteristics of the voltage-controlled oscillation circuit are deteriorated.
In order to overcome the problems described above, preferred embodiments of the present invention provide a voltage-controlled oscillator having a greatly reduced size and a structure that prevents characteristic deterioration. In addition, preferred embodiments of the present invention provide a communication apparatus including a voltage-controlled oscillator having a greatly reduced size and a structure that prevents characteristic deterioration.
A voltage-controlled oscillator according to a preferred embodiment of the present invention includes a first resonant circuit resonating in a first frequency band, a second resonant circuit resonating in a second frequency band that is higher than the first frequency band, an oscillation circuit oscillating at respective resonance frequencies of each of the first and second resonant circuits, and an amplifying circuit amplifying an oscillation signal transmitted from the oscillation circuit. In this oscillator, the first resonant circuit has high impedance in the second frequency band.
In addition, the first resonant circuit may be a series resonant circuit including a variable-capacitance diode and a resonator, and the second resonant circuit may be a parallel resonant circuit including a variable-capacitance diode and a resonator.
In addition, each of the first and second resonant circuits may be a series resonant circuit including a variable-capacitance diode and a resonator.
Another preferred embodiment of the present invention provides a communication apparatus incorporating the novel voltage-controlled oscillator described above.
In the voltage-controlled oscillator according to preferred embodiments of the present invention, since the first resonant circuit maintains high impedance in the second frequency band permitting the oscillation of the second resonant circuit, this unique arrangement prevents the first resonant circuit from imposing a load on the second resonant circuit in the second frequency band.
In the communication apparatus of other preferred embodiments of the present invention, since the voltage-controlled oscillator is capable of preventing the characteristic deterioration in the first and second frequency bands, the communication apparatus achieves excellent communication characteristics both in the first and second frequency bands.
Other features, elements, characteristics, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments of the present invention with reference to the attached drawings.